1. Field of the Invention
The present invention relates to a packet communication system and a packet communication apparatus adapted to control time delay variation of packet transmission.
2. Description of the Related Art
Two conventional techniques have been known for controlling time delay variation of packet transmission. They are a leaky bucket technique and a time-stamp technique. The leaky bucket technique involves having packets stored temporarily in a receiving-side buffer so that the packets are output onto a transmission path at an average packet rate, whereby time delay variation of packet transmission is controlled.
An improved version of the leaky bucket technique is what is known as a dynamic rate leaky bucket technique. This technique is designed to vary the output rate of the leaky bucket technique in keeping with a packet input rate. An example of the dynamic rate leaky bucket technique is disclosed in xe2x80x9cPerformance of Dynamic Rate Leaky Bucket Algorithm,xe2x80x9d Electronics Letters, pp. 1560-1561, Aug. 19, 1993, Vol. 29, No. 17.
The time-stamp technique is a technique which records in packet headers the times at which packets are input to a packet transmitting device so that a receiving side outputs these packets onto a transmission path in accordance with the times recorded in their headers, whereby time delay variation of packet transmission is controlled. This technique is characterized in that packet input time information is attached to each of the packets to be transmitted. An example of the time-stamp technique is disclosed in xe2x80x9cA Cell Stream Multiplex Scheme Employing Time-Stamp Techniques for Wireless ATM,xe2x80x9d Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), IN97-27, CS97-8, MVE98-8 (1997-4).
One disadvantage of the leaky bucket technique is its low level of temporal accuracy in time delay control. That is because packets are kept output at a constant rate from a receiving-side buffer whether the rate of packets input to a packet-transmitting side is increased or decreased.
One disadvantage of the time-stamp technique is its low level of packet transmission efficiency because of an overhead brought about when a large amount of time information is attached to each packet to enhance control time accuracy. Since reference time intervals at which to record time information remain constant for a given system, it is difficult to change the temporal accuracy in time delay variation in a flexible and simplified manner. Furthermore, the conventional techniques above are each constituted to have a delay time added at the receiving-side buffer in order to absorb time delay variation.
With any of the above techniques, once the time delay variation of packet transmission exceeds a tolerable range of absorption by the receiving-side buffer, the phenomenon called a buffer underflow occurs. That is, packets to be output have yet to arrive at the receiving side when the times at which to send them out have already passed.
A solution to the underflow problem would be to establish an increased delay variation time for absorption by the buffer. This method, if implemented, will promote delays at the buffer. To inhibit underflows without increased delays at the buffer requires minimizing the amount of time delay variation occurring in packet transmission sections.
It is therefore an object of the present invention to provide a packet communication system and a packet communication apparatus for minimizing the amount of time delay variation occurring in packet transmission sections.
In carrying out the invention and according to a first aspect thereof, there is provided a packet communication system comprising: a first communication apparatus for transmitting input time information about input packets before transmitting the packets; and a second communication apparatus for receiving the input time information about the packets and transmitting the packets over a transmission path in an order based on the input time information.
Preferably, the first communication apparatus may perform packet transmission in units of frames, each frame being composed of at least one packet. The input time information about the packets may be constituted by at least one of an input time and an input time interval for each of the packets input to the first communication apparatus. The second communication apparatus may transmit the packets over the transmission path in chronological order of the input times of the packets input to the first communication apparatus. The invention may also be applied where the packets are made of asynchronous transmission mode (ATM) cells.
The packet communication system of this invention is formed specifically by a wireless base station and one or a plurality of wireless terminal stations. Thus according to a second aspect of the invention, there is provided a wireless base station comprising: an interface for receiving a signal; a signal processing unit for attaching input time information to the signal; an access processing unit for transmitting the input time information before transmission of the signal; and a controller for assigning bandwidths at predetermined intervals. And according to a third aspect of the invention, there is provided a wireless terminal station comprising: an interface for receiving a signal; a signal processing unit for attaching input time information to the signal; and an access processing unit for transmitting the input time information before transmission of the signal.
According to a fourth aspect of the invention, there is provided a packet communication system comprising: a first communication apparatus for attaching to a frame input time information about at least one packet constituting the frame to be transmitted; and a second communication apparatus for receiving the frame to which the input time information about the packets was attached and for transmitting the packets over a transmission path in accordance with the input time information. Preferably, the input time information about the packets may be thinned out before being attached to the frame.
According to a fifth aspect of the invention, there is provided a packet communication system comprising: a first communication apparatus for attaching to a frame input time information about part of a plurality of packets constituting the frame to be transmitted; and a second communication apparatus for transmitting the packets having the input time information attached to the received frame over a transmission path in accordance with the input time information, the second communication apparatus further transmitting the other packets with no input time information attached to the frame over the transmission path at predetermined time intervals.
Preferably, the predetermined time intervals may be constant time intervals. Alternatively, the predetermined time intervals may be obtained by linearly approximating either time changes of packet input time intervals or a packet input rate.
As outlined, information about input times or input time intervals of packets (ATM cells) that are input to a packet communication apparatus is transmitted to a receiving-side packet communication apparatus before transmission of the packets. In the receiving-side packet communication apparatus, terminal equipment having the packets that were input the earliest to the transmitting-side packet communication apparatus gains preferentially the earliest access to the transmission medium. This scheme minimizes time delay variation of packet transmission in packet transmission sections, reducing the possibility of underflows and suppressing overflows in a buffer of a destination frame transmitter-receiver. Because the destination frame transmitter-receiver can control time delay variation of packet transmission in accordance with the input times or input rates of packets (ATM cells) that were input to a source frame transmitter-receiver, it is possible to attach to each packet its time-related information and to change packet output rates on the frame-receiving side in a flexible and simplified manner.